Irradiation methods in particle beam therapy systems mainly include a broad irradiation method in which a charged particle beam is enlarged in a dispersion manner by a scatterer, the shape of the enlarged charged particle beam is made to coincide with the shape of an irradiation subject in order to form an irradiation field, and then the beam is irradiated all at once onto the whole diseased site of a patient as an irradiation subject; and a scanning irradiation method (the spot-scanning method, the raster-scanning method, and the like) in which a thin, a pencil-like beam is irradiated by scanning the beam with a scanning electromagnet in such a way that the scanning area coincides with the shape of an irradiation subject.
In recent years, in order to treat a complex-shape diseased site, the demand for the degree of freedom in forming a beam has become large. A brain tumor is an example of these diseased sites. Because being surrounded by major organs onto which any beam should not be irradiated, a brain tumor has a complex shape. The broad irradiation method is not suitable for the treatment of this kind of diseased site. The reason for that is as follows. In the broad irradiation method, a beam is spread in the three dimensions, and the unnecessary part thereof is eliminated by use of a collimator or a bolus; then, an irradiation field is formed in such a way as to coincide with the shape of a diseased site. In the broad irradiation method, in the case where a complex-shape diseased site is treated, it is difficult to form a complex-shape irradiation field only with one-time irradiation. Therefore, a method should be utilized in which irradiations from various directions are superimposed. Moreover, because irradiations from various directions are superimposed, it is difficult to correctly superimpose these irradiations; thus, unevenness in the beam irradiation amount may be caused. Still moreover, because a beam has a certain spread, an unnecessary beam may be irradiated onto a normal tissue in the vicinity of the place where irradiations are superimposed.
In contrast, in the scanning irradiation method, a diseased site is divided into small spots in the three dimensional space and respective necessary-amount beams are irradiated onto the small spots so that an irradiation field is formed in accordance with the shape of the whole diseased site; therefore, in principle, by selecting the spots, the scanning irradiation method can be applied to any diseased site shape; thus, the scanning irradiation method is an irradiation method having such a high degree of freedom that no collimator or bolus is required. Moreover, the amount of a beam to be irradiated can be adjusted for each spot; therefore, even in the case where irradiations from various directions are superimposed, the beam amount at the place where the irradiations are superimposed can be reduced. However, because a collimator or a bolus for preventing irradiation onto normal tissues other than the diseased site is not utilized, a high accuracy of the irradiation position is required. That is to say, there is required an irradiation-position accuracy that is higher than that required in a broad irradiation method.
Patent Document 1 discloses an invention in which in a particle beam therapy system utilizing a scanning irradiation method that requires a high accuracy in the beam irradiation position, an obstacle that causes beam dispersion is placed at a position that is as downstream in the beam as possible so that the beam size is reduced. The invention disclosed in Patent Document 1 is provided with a beam scanning apparatus that scans a charged particle beam, a first duct in which a beam extracting window is provided at a position that is at the downstream side of the beam scanning apparatus, an irradiation apparatus that makes a charged particle beam pass through the first duct and that irradiates the charged particle beam onto an irradiation subject, a second duct, and a beam transport apparatus that makes a charged particle beam, launched from an accelerator, pass through the second duct and that transports the charged particle beam to the irradiation apparatus; a beam position monitor (referred to simply as a position monitor, hereinafter) that measures the position of a charged particle beam is mounted in the beam extracting window through the intermediary of a holding member; a vacuum region in the first duct and a vacuum region in the second duct communicate with each other.
By use of a duct driving means and a duct extension/contraction means that extends and contracts the first duct in the beam-axis direction, the position monitor, which is provided at a position that is in the vicinity of and at the downstream side of the beam extracting window, is moved while the first duct moves in the beam-axis direction, so that the air gap between a patient and the beam extracting window is suppressed from becoming unnecessarily large and hence the beam size is reduced.